1. Field of the Invention
The invention in general relates to the field of water-flow indicators that are intended to be mounted on a water-pipe of sprinkler-type fire suppressant systems to activate an alarm when water flows in the pipe. In particular, it relates to such water-flow indicators having a time delay function.
2. Description of the Prior Art
Water-flow detectors, sometimes referred to as water-flow indicators, which are mounted on the pipes of sprinkler-type fire suppressant systems are well-known. These systems generally include a paddle or vane which is placed within the pipe or other conduit carrying the water. The vane is connected to a pivot arm which connects to an alarm actuator. A problem with such water-flow detectors is that transient flows can occur in the sprinkler system pipes; for example, when a pump that maintains pressure in the system turns on, air trapped in the system will be compressed, which may cause water to temporarily surge past the flow detector location. Such temporary surges can trigger false alarms which are a nuisance. The common way to avoid such nuisance alarms is to provide a time delay between the beginning of the flow and signaling an alarm. Typically, an adjustable pneumatic dashpot (similar to that of a screen door closer) retard mechanism is used to provide the delay.
The prior art delay mechanisms have a number of disadvantages. The pneumatic retard is vulnerable to air borne particles which can clog the miniscule exhaust path and significantly alter the delay or even prevent the actuation of the alarm. The mechanical linkages can vary or deteriorate to cause significant variation in the force required to actuate the mechanism, thus altering the flow sensitivity of the detector. Further, setting the pneumatic retard time delay involves a time-consuming trial-and-error method since absolute calibration is difficult with such retards.